Oldest Evidence for Complex Life in Doubt

Life on Earth is thought to have developed from a common ancestor. Now scientists have determined that the three known domains of life – eukaryota, bacteria and archaea – had already diverged from one another by 2.7 billion years ago.

Chemical biomarkers in ancient Australian rocks, once thought to be the oldest known evidence of complex life on Earth, may have infiltrated long after the sediments were laid down, new analyses suggest.

The evidence was based on biomarkers – distinctive chemical compounds produced today by modern-day relatives of cyanobacteria and other complex life forms. In 1999, a team of researchers contended that the biomarkers in the 2.7-billion–year-old rocks pushed back the origins of cyanobacteria by at least 550 million years and of eukaryotes by about a billion years.

Although some scientists interpret the new findings, published in the Oct. 23 Nature, as disproving the older dates, others contend that the results still allow for the presence of the organisms or their kin at that time.

Experts believe that the first life on Earth consisted of single-celled organisms, or prokaryotes, such as bacteria. Later came the cyanobacteria, the group of photosynthesizing bacteria that produce oxygen. Before the 1999 work, the oldest known fossils of this group were about 2.15 billion years old. Likewise, the oldest fossils of eukaryotes, the group whose cells contain nuclei, were about 1.7 billion years old. The remains of any of these organisms can be destroyed when intense heat and pressure deep within Earth convert the molecules into petroleum and kerogen, a mixture of long-chain, carbon-rich compounds.

Results of the first analyses of the Australian rocks were controversial, says Birger Rasmussen, a geochemist at Curtin University of Technology’s campus in Bentley, Australia. For one thing, the shale – which had been laid down as sediments about 2.7 billion years ago – contained tiny particles of pyrobitumen, coal-like remnants of oil droplets that had solidified as the sediment layers cooked. Pyrobitumen is a sign that the sediments and any organic material they contained experienced temperatures from 200° Celsius to 300°C for an extended time. The rocks also contained significant amounts of kerogen.

Yet the samples also held small quantities of hopanes, a class of organic chemicals produced by cyanobacteria and some other bacteria, as well as steranes, which are produced only by eukaryotes. That the rocks hosted these biomarkers, which should have been destroyed by the heat and pressure required to generate the pyrobitumen, "presented a bit of a conundrum," Rasmussen says.

However, because the Australian rocks otherwise showed little evidence of heat-driven degradation, scientists at the time largely dismissed the notion that the hopanes and steranes had migrated into the rocks many years after the sediments had formed. But that interpretation led to yet another conundrum — the inferred presence of oxygen-generating cyanobacteria at least 350 million years before significant amounts of oxygen showed up in the atmosphere.

Cyanobacteria (above) became the first microbes to produce oxygen by photosynthesis.
Credit:UC Berkeley

The tests reported in 1999, and others conducted since then, compared the ratios of carbon isotopes in the biomarkers, which could be extracted from the strata, with those of the pyrobitumen and kerogen left trapped in the rocks. Comparing those ratios enables researchers to determine whether the two derived from the same batch of organic material.

Previous isotopic measurements were analyzed in bulk, so they couldn’t distinguish isotope ratios in the pyrobitumen from those of the kerogen nearby, says Ian Fletcher, also of the Curtin University of Technology and a coauthor of the new report. In the new analyses, however, he, Rasmussen and their colleagues – including one scientist who was part of the 1999 team – used an instrument with a microprobe that enabled them to measure the carbon-isotope ratios on spots of intact rock as small as 5 micrometers across, about the size of a single bacterium.

Typically, the proportion of the carbon-13 isotope found in kerogen and other hydrocarbons is between 1 and 3 parts per thousand less than the proportion found in the original organic matter from which those substances derived, Fletcher says. But the team’s new analyses show that the proportions of carbon-13 isotopes of the kerogen and pyrobitumen in the Australian rocks is between 10 and 20 parts per thousand less than those found in the hopanes and steranes – the presumably unmodified biomarkers for cyanobacteria and eukaryotes – extracted from the same rocks. This difference indicates that the kerogen and the pyrobitumen are probably unrelated to the biomarkers, says Fletcher. It’s also a strong sign that the biomarkers migrated into the rocks sometime after 2.2 billion years ago, when the rocks underwent most of the metamorphosis that formed pyrobitumen. "We can’t say where the biomarkers came from, or when," he notes. "We can’t pretend to know."

Instruments that can detect substances present in concentrations as small as a few parts per billion pose a challenge because the sensors can also pick up trace contaminants, comments Woodward Fischer, a geobiologist at Caltech in Pasadena. Biomarkers such as hopanes are produced by organisms but are also detected in diesel exhaust, fossil fuel emissions and urban air pollution.

Ancient evidence of life has also been found in rocks like the one shown above, which was recovered in South Africa. Coin at left shows scale.
Credit:Penn State

The new findings "are interesting observations but Rasmussen and his colleagues pose only one explanation for the results," says Jennifer Eigenbrode, an organic geochemist at NASA Goddard Space Flight Center in Greenbelt, Md.

While some of the organisms in microbial communities 2.7 billion years ago got carbon from carbon dioxide, others took it from methane. Both gases were abundant in the oxygen-free atmosphere at that time. "It was a completely different planet then," Eigenbrode says.

Because those microbes often recycled carbon among themselves many times, the proportions of carbon-13 in their biomarkers ranged more widely than they do across most microbial communities today.

It’s possible that a community of microbes could make hopanes and steranes that, when cooked, would produce the kind of isotopically light pyrobitumen and kerogen seen in the Australian rocks. So the biomarkers could still be legitimate indicators of early complex life.

Since the 1999 study of the 2.7-billion-year-old Australian rocks, other analyses of rocks of similar age elsewhere in the world, particularly some from South Africa, also have detected biomarkers that bolster the early appearance of cyanobacteria, says Andrew H. Knoll, a biogeochemist at Harvard University. Even if the new results "take some specific evidence off the table, all is not lost," he says.